A primary problem which must be addressed in order to improve the performance and longevity of gas turbine engines is that of oxidation of the high temperature turbine components. The structural superalloy materials employed for gas turbine components have some degree of inherent oxidation resistance, however, the compositional compromises necessary to achieve the ultimate in mechanical properties generally result in the reduction of the materials' oxidation resistance to below that which is required for long-term use. In addition, the trend in gas turbine engines is always towards increasing temperatures to improve performance and efficiency. The oxidation rate of materials increases dramatically with increasing temperature.
For these reasons, for at least twenty years it has been customary to use protective coatings on gas turbine engine components. Such coatings are two general types, the aluminide type and the overlay type. Aluminide-type coatings are produced by diffusing aluminum into the surface of the component at elevated temperature in order to provide an aluminum rich surface zone which increases the material's oxidation resistance by providing sufficient aluminum to develop a protective alumina scale and providing sufficient aluminum to reform this scale as it spalls from the surface under use conditions. The aluminide-type coatings are generally quite thin and are life limited by the further diffusion of the aluminum into the component and by spallation of the aluminum oxide surface scale, both of which ultimately reduce the surface aluminum level to below the level which will form an alumina surface scale. However, such coatings are desirable for high performance engines inasmuch as the components coated with aluminide coatings are found to have substantially enhanced resistance to thermal fatigue when compared with overlay coated parts.
Overlay-type coatings consist of a discrete layer of an oxidation resistance alloy which is deposited on the surface of the component by means which include, for example, vapor deposition or plasma spraying. Overlay coatings are generally developed to be inherently oxidation resistant but they are developed in large measure without much consideration of the substrate to which they are to be applied. Thus, for example, a single overlay coating composition may be applied to many different composition substrates. Of course, such overlay coatings are optimized for oxidation resistance by more or less trial and error approach in which coated samples are tested and the coating then modified and retested. However, such trial and error techniques start from an arbitrary oxidation resistant material baseline rather than from the substrate composition.